\documentclass{article}

\usepackage{agda}

% Redefining \AgdaIndent as a one-argument command should work.
\renewcommand{\AgdaIndent}[1]{~~}

\begin{document}

\begin{code}%
\>[0]\AgdaKeyword{postulate}\<%
\\
\>[0][@{}l@{\AgdaIndent{0}}]%
\>[2]\AgdaPostulate{Whatever}\AgdaSpace{}%
\AgdaSymbol{:}\AgdaSpace{}%
\AgdaPrimitive{Set}\<%
\end{code}

\end{document}
